Conventional techniques for locating RF emitters such as wireless access points and laptops with IEEE 802.11 capability and other such RF emitters are based on measuring the amplitude of the emitter with a portable receiver, and moving around to find the direction in which the amplitude increases. The general assumption is that the stronger the signal amplitude, the closer the emitter is believed to be. Several commercial locating devices have been developed for this purpose (e.g., Yellowjacket® 802.11b Wi-Fi Analysis System).
There are a number of problems associated with such amplitude-based techniques for locating emitters. For instance, the techniques tend to be highly inaccurate due to the incidence of RF multipath created by the RF waveforms emanating from the 802.11 and other such RF emitters. These waveforms bounce off conductive objects or surfaces in the environment, which in turn cause multiple false readings on increased amplitude (false directions) that subsequently disappear as the user leaves the multipath. Thus, conventional amplitude-based locating techniques can create false high amplitude paths to the target and will not work in a high multipath environment such as a neighborhood (e.g., street scene) or building (e.g., home, office building, or café). Moreover, lower power RF signals of interest are difficult to detect and locate in the presence of relatively strong interference.
There is a need, therefore, for techniques that allow for the detection and locating of RF emitters, and particularly low-power RF emitters in the presence of interference.